docs: def add_cache_tier(self, cache_pool, mode):
          """
          Adds a new cache tier to an existing pool.
          :param cache_pool: six.string_types.  The cache tier pool name to add.
          :param mode: six.string_types. The caching mode to use for this pool.  valid range = ["readonly", "writeback"]
          :return: None
          """
          # Check the input types and values
          validator(value=cache_pool, valid_type=six.string_types)
          validator(value=mode, valid_type=six.string_types, valid_range=["readonly", "writeback"])
  
          check_call(['ceph', '--id', self.service, 'osd', 'tier', 'add', self.name, cache_pool])
          check_call(['ceph', '--id', self.service, 'osd', 'tier', 'cache-mode', cache_pool, mode])
          check_call(['ceph', '--id', self.service, 'osd', 'tier', 'set-overlay', self.name, cache_pool])
          check_call(['ceph', '--id', self.service, 'osd', 'pool', 'set', cache_pool, 'hit_set_type', 'bloom'])